There are two types of pigment that give hair its color, eumelanin(black) and pheomelanin(red). All humans have pheomelanin in their hair. How dark it is depends on how much eumelanin is present. A low concentration of eumelanin in the hair will give blonde hair, more eumelanin will give it a brown color, and much higher amounts of eumelanin will result in black hair. Eumelanin in low concentrations causes a yellow tone, in higher concentrations creates a brown color. Also, in general, the more melanin present, the darker the hair color; less melanin, the lighter the hair color. Pheomelanin is more chemically stable than eumelanin, so it breaks down more slowly when oxidized. Bleach will cause darker hair to turn red as it is processing because it has broken down the eumelanin quickly but acts more slowly on the pheomelanin. As the pheomelanin breaks down, the hair will then become orange, and then the chemicals turn it yellow.
All “permanent” hair color products and lighteners contain both a developer, or, oxidizing agent, and an alkalizing ingredient as part of their ammonia or an ammonia substitute. The purpose of this is to raise the cuticle of the hair fiber so the tint can penetrate, facilitate the formation of tints within the hair fiber, and bring about the lightening action of peroxide. When the tint containing the alkalizing ingredient is combined with the developer (usually hydrogen peroxide), the developer becomes alkaline and diffuses through the hair fiber, entering the cortex, where the melanin is located. The lightening occurs when the alkaline peroxide breaks up the melanin and replaces it with new color.
The outer layer of the hair shaft, its cuticle, must be opened before permanent color can be deposited into the hair. Once the cuticle is open, the dye reacts with the inner portion of the hair, the cortex, to deposit or remove the color. Most permanent hair colors use a two-step process (usually occurring simultaneously) which first removes the original color of the hair and then deposits a new color. It's essentially the same process as lightening, except a colorant is then bonded within the hair shaft. Ammonia is the alkaline chemical that opens the cuticle and allows the hair color to penetrate the cortex of the hair. It also acts as a catalyst when the permanent hair color comes together with the peroxide. Peroxide is used as the developer or oxidizing agent. The developer removes pre-existing color. Peroxide breaks chemical bonds in hair, releasing sulfur, which accounts for the characteristic odor of hair color. As the melanin is decolorized, a new permanent color is bonded to the hair cortex. Various types of alcohols and conditioners may also be present in hair color. The conditioners close the cuticle after coloring to seal in and protect the new color.
About 80% of hair consists of elongated cells (cortical cells) of a fibroid structure (macro-fibrils, micro-fibrils). The cortex determines the thickness, elasticity, and strength of the hair. It is also responsible for housing all of the hairs' natural color pigments. Color pigments found in the cortical layer are in the form of minute melanin granules. These granulized color pigments are stored in tiny sacks called alveoli. In healthy hair, light can penetrate the cuticle layer and reflect directly off the melanin pigment in the hair with little refraction, thus producing greater luminosity, and subjectively, truer, more vibrant color. When the cuticle is damaged, refraction convolutes the natural balance of color and the luminosity of the reflected light, resulting in a dull, muddied color.
Problems in using current hair treating formulations include compromised cuticle, non-uniform distribution of pigment, and surface bonding of pigment in coloring. In addition, the distribution of the pigment granules within the structure of the keratin surface will not reflect light uniformly because of the refracted light, causing a dull color that lacks a natural luster. The compromised cuticle also leaves the surface bonded pigment vulnerable to decay because it is now exposed more easily to the elements
Ammonia (or other basic materials of pH>8) is used in permanent (oxidative) hair color. When the permanent hair color and the developers come together, the action of ammonia begins. Like all alkaline materials, the ammonia has the tendency to separate the cuticle and allows the permanent hair color to penetrate the cortex of the hair. The ammonia has an effect on the sulfur bonds of the hair. If the ammonia is too harsh, thus penetrating too deeply for too long, the hair will lose more of the sulfur bonds than necessary. It will cause the hair to harden, lose weight and diameter. The higher the volume of the developer, the greater the amount of sulfur is removed from the hair structure. This is one of the reasons why developers are maintained at 30% volume or less for the majority of hair coloring.
Many people believe that continual styling over time and/or heat damages the hair, but neither continual styling nor heat as such is damaging. It is over-styling and heat in the absence of moisture that damages hair. Styling for too long may reduce moisture to unhealthy levels. Once essential moisture in hair has been removed, the hair becomes extremely vulnerable to temperature change in convective and conductive situations. Styling under these conditions will cause hair to lose its elasticity and make the cuticle vulnerable to barbing, opening, electrostatic effects, etc. Damage to the cuticle of the hair will make it more breakable and will make pigment vulnerable to chemical reactions, decay, and diffusion. Too much heat without significant moisture will more easily create steam inside the hair shaft which in turn will burst hair by breaking it.